3,6-disubstituted 2-halopyridines

ABSTRACT

The invention relates to halopyridines of the formula I ##STR1## in which L is Cl or F, and 
     R 1 , R 2 , A 1 , A 2 , Z 1 , Z 2 , m and n are as defined in patent claim 1, and the use thereof as components of liquid-crystalline, in particular ferroelectric, media.

FIELD OF THE INVENTION

The invention relates to 3,6-disubstituted 2-halopyridines of theformula I ##STR2## where R¹ and R² are each, independently of oneanother, an alkyl or alkenyl radical having up to 15 carbon atoms whichis unsubstituted or monosubstituted by CN, halogen or CF₃, it also beingpossible for one or more CH₂ groups in these radicals to be replaced, ineach case independently of one another, by --S--, --O--,--CO--,--CO--O--,--O--CO-- or --O--CO--O-- in such a manner that Sand/or O atoms are not linked directly to one another, and one of theradicals R¹ and R² is alternatively halogen, --CF₃, --OCF₃ or OCHF₂,

A¹ and A² are each, independently of one another, a

(a) trans-1,4-cyclohexylene radical in which, in addition, one or morenon-adjacent CH₂ groups may be replaced by --O-- and/or --S--,

(b) 1,4-phenylene radical in which, in addition, one or two CH groupsmay be replaced by N,

(c) radical from the group comprising 1,3-cyclobutylene,1,3-bicyclo[1.1.1]pentylene, 1,4-cylohexenylene [sic], 1,4-bicyclo[2.2.2]octylene, piperidine-1,4-diyl, naphthalene-2,6-diyl,decahydronaphthalene-2,6-diyl and1,2,3,4-tetrahydronaphthalene-2,6-diyl,

it being possible for the radicals (a) and (b) to be substituted by CNor halogen,

Z¹ and Z² are each, independently of one another, --CH₂ --CH₂ --,--C--C--, --CH₂ O--, --OCH₂ --, --CO--O--, --O--CO--, --CH═N--, --CH₂S--, --SCH₂ --, a single bond or an alkylene group having 3 to 6 carbonatoms in which, in addition, one CH₂ group may be replaced by--O--,--CO--O--, --O--CO--, --CH-halogen- or --CHCN--,

L is F or Cl,

m is 0, 1 or 2,

n is 0, 1 or 2, and

m+n is 1, 2 or 3,

with the provisos that

a) in the case where L=F and R² is halogen, --CF₃, --OCF₃, OCHF₂ oralkoxy, n is 1 or 2, and/or

b) in the case where L=F and n=1 or 2,

Z² is --CH₂ CH₂ --, --C--C--, --CH₂ O--, --CO--O-- or a single bond.

BACKGROUND OF THE INVENTION

Chiral tilted smectic liquid-crystalline phases having ferroelectricproperties can be prepared by adding a suitable chiral dope to basemixtures having one or more tilted smectic phases (L. A. Beresnev etal., Mol. Cryst. Liq. Cryst. 89, 327 (1982); H. R. Brand et al., J.Physique 44 (lett.), L 771 (1983). Phases of this type can be used asdielectrics for fast-switching displays based on the principle of SSFLCtechnology described by Clark and Lagerwall (N. A. Clark and S. T.Lagerwall, Appl. Phys. Lett. 36, 899 (1980); U.S. Pat. No. 4,367,924) onthe basis of the ferroelectric properties of the chirally [sic] tiltedphase. The elongate molecules in this phase are arranged in layers, themolecules having a tilt angle to the layer normals. On moving from layerto layer, the tilt direction changes by a small angle with respect to anaxis perpendicular to the layers, so that a helical structure is formed.In displays based on the principle of SSFLC technology, the smecticlayers are arranged perpendicular to the plates of the cell. The helicalarrangement of the tilt directions of the molecules is suppressed by avery small separation of the plates (about 1-2 μm). This forces thelongitudinal axes of the molecules to line up in a plane parallel to theplates of the cell, which causes two preferential tilt alignments to beformed. Application of a suitable alternating electrical field allowsswitching back and forth between these two states in theliquid-crystalline phase having spontaneous polarization. This switchingoperation is significantly faster than in conventional twisted cells(TN-LCDs), which are based on nematic liquid crystals.

The currently available materials having chirally [sic] tilted smecticphases (such as, for example, Sc* [sic ], but also S_(H) *, S_(I) *,S_(J) *, S_(K) *, S_(G) * and S_(F) *) has the great disadvantage formany applications of low chemical, thermal and photostability. A furtherdisadvantageous property of displays based on currently availablechirally [sic] tilted smectic mixtures is that the spontaneouspolarization has values which are too low, so that the response timebehavior of the displays is adversely affected and/or the pitch and/orthe tilt and/or the viscosity of the phases does not meet therequirements of display technology. In addition, the temperature rangeof the ferroelectric phases is usually too small and is predominantly atexcessively high temperatures.

SUMMARY OF THE INVENTION

It has now been found that the use of compounds of the formula I ascomponents of chirally [sic] tilted smectic mixtures can significantlyreduce said disadvantages. The compounds of the formula I are thuseminently suitable as components of chirally [sic] tilted smecticliquid-crystalline phases. They can be used, in particular, to preparechirally [sic] tilted smectic liquid-crystalline phases which areparticularly stable chemically and have favorable ferroelectric phaseranges, favorable distances [sic] for the viscosity, in particularhaving broad Sc* [sic] phase ranges, excellent supercoolability down totemperatures below 0° C. without crystallization occurring, and valuesfor the spontaneous polarization which are moderate to high for phasesof this type. P is the spontaneous polarization in nC/cm². However, thecompounds of the formula I are also suitable for liquid-crystallineS_(A) phases for the electroclinic effect.

The compounds of the formula I have negative anisotropy of thedielectric constants (Δε=-0.2 to -5.0) at the same time as a highdielectric constant perpendicular to the longitudinal axis of themolecule (ε.sub.⊥) [lacuna] therefore have a broad range ofapplications. Depending on the choice of substituents, these compoundscan be used as base materials from which liquid-crystalline smecticphases are predominantly composed; however, compounds of the formula Ican also be added to liquid-crystalline base materials from otherclasses of compound in order, for example, to vary the dielectric and/oroptical anisotropy and/or the viscosity and/or the spontaneouspolarization and/or the phase range and/or the tilt angle and/or thepitch of a dielectric of this type.

Similar compounds are disclosed, for example, in the internationalPatent Application WO 88/07992. These contain a2-chloro-3-cyanopyridin-6-yl group, are employed as intermediates in thepreparation of compounds containing a 4-cyanopyridin-6-yl group and arenot suitable as components of chirally [sic] tilted, smecticliquid-crystalline media.

The compounds excluded by provisos a) and b) are the subject-matter ofthe international Application PCT/EP 90/01536, which is not a priorpublication. These are not suitable for ferroelectric displays.

The optically active compounds of the formula I are covered by thegeneral formula, for example, of EP-0 283 326-A. However, a personskilled in the art is able to deduce from this application neither theadvantageous properties of the compounds according to the invention normethods for their preparation.

A person skilled in the art was thus able to deduce from the prior artin a simple manner neither possible syntheses for the compounds claimednor that the compounds according to the invention have predominantlybroad and favorably located S_(c) phases and are distinguished byfavorable values for the rotational viscosity.

The invention thus relates to the 2-halopyridines of the formula I, inparticular in which n is 1 or 2, and at least one of the radicals A¹ andA² is optionally fluorine-substituted 1,4-phenylene, 1,4-cyclohexylene,pyrimidine-2,5-diyl or pyridine-2,5-diyl.

The compounds of the formula I include the bicyclic compounds of theformula [sic] I1-I4 ##STR3## in which R¹, R¹ 2, A¹, A¹, Z¹ and Z², areas defined above, and

R², is unsubstituted alkyl, oxaalkyl or dioxaalkyl, or alkyl or alkoxywhich is monosubstituted by CN, halogen or CF₃, and

Z² is --CH₂ CH₂ --, --C--C, --CH₂ O--, --CO--O-- or a single bond,tricyclic compounds of the formulae I5 to I10 ##STR4## and tetracycliccompounds of the formula Ill to I12. ##STR5##

The invention furthermore relates to a process for the preparation ofthe compounds of the formula IZ ##STR6## in which X' is H, Cl, alkyl,alkoxy, oxaalkyl or dioxaalkyl having 1 to 12 carbon atoms,

characterized in that 2-chloro- or 2,6-dichloropyridine is treated in aninert solvent with a strong base, preferably lithium diisopropylamide,subsequently reacted with trimethyl borate and acidified.

The invention furthermore relates to the compounds of the formula IZ.

The compounds are highly suitable for the preparation of the compoundsof the formula I (cf. Schemes 1 and 3).

The invention also relates to the use of the compounds of the formula Iin which L is Cl for the preparation of the compounds of the formula II,##STR7## in which R¹, R², A¹, A², Z¹, Z¹, m and n are as defined forformula I.

The compounds of the formula II are known per se (for example WO87/04158), but the processes described hitherto only result inunsatisfactory yields.

To prepare the compounds of formula II, the compounds of the formula Iin which L is Cl are treated in an inert solvent with complex hydridesor hydrogenated in the presence of a transition-metal catalyst.

The invention relates, in particular, to optically active halopyridinesof the formula I in which one of the radicals R¹ and R² is a chiralgroup of the formula III ##STR8## where R⁴ is a group of the formula(CH₂)_(s) --Q³ --C_(o) H_(2o+1), in which

Q³ is --O--, --O--CO-- or a single bond,

s is 0, 1 or 2, and

o is 1 to 7,

Y is CN, halogen or CH₃,

Z is a single bond or --(CH₂)_(p) -- in which one CH₂ group may bereplaced by --O--, --O--CO-- or --CO--O--, and p is 1, 2, 3,-4, 5 or 6,and

R^(o) is H or CH₃,

with the proviso that only one of the groups R^(o), R⁴ and Y is CH₃. Theinvention furthermore relates to liquid-crystalline phases which containat least one compound of the formula I. The invention moreover relatesto ferroelectric liquid-crystalline phases containing at least onecompound which [lacuna] a group of the formula [lacuna] ##STR9## of theformula I and liquid-crystal display elements, in particularferroelectric electrooptical display elements, which contain phases ofthis type.

The phases according to the invention preferably contain at least two,in particular at least three, compounds of the formula I. Particularpreference is given to chiral tilted smectic liquid-crystalline phasesaccording to the invention whose achiral base mixture, in addition tocompounds of the formula I, contains at least one other component havingnegative or small positive dielectric anisotropy. This further componentor these further components of the achiral base mixture can make up from1 to 50%, preferably from 10 to 25%, of the base mixture. Suitablefurther components having small positive or negative dielectricanisotropy are compounds of the formula IV, which include the compoundsof the subformulae IVa to IVi: ##STR10##

R⁴ and R⁵ are each preferably straight-chain alkyl, alkoxy, alkanoyloxyor alkoxycarbonyl, in each case having 3 to 12 carbon atoms. X ispreferably O. In the compounds of the formulae IVa, IVb, IVd, IVe, IVfand IVg, it is also possible for one 1,4-phenylene group to be laterallysubstituted by halogen or CN, particularly preferably by fluorine.

Particular preference is given to the compounds of the subformulae IVa,IVb, IVd and IVf in which R⁴ and R⁵ are each straight-chain alkyl oralkoxy, in each case having 5 to 10 carbon atoms. Particularly preferredindividual compounds are indicated in Table I below:

                  TABLE I                                                         ______________________________________                                        Formula   R.sup.4       R.sup.5   X                                           ______________________________________                                        IVa       n-decyloxy    n-heptyloxy                                                                             0                                           IVa       n-hexyloxy    n-decyloxy                                                                              0                                           IVa       n-octyloxy    n-heptyl  0                                           IVa       n-octyloxy    n-pentyl  0                                           IVa       n-decyloxy    n-heptyl  0                                           IVa       n-decyloxy    n-pentyl  0                                           IVf       n-pentyl      n-pentyl  0                                           IVf       n-pentyl      n-hexyl   0                                           ______________________________________                                    

The compounds of the subformulae IVc, IVh and IVi are suitable asadditives for reducing the melting point and are normally added to thebase mixtures in an amount of not more than 5%, preferably from 1 to 3%.R⁴ and R⁵ in the compounds of the subformulae IVc, IVh and IVi arepreferably straight-chain alkyl having 2 to 7, preferably 3 to 5, carbonatoms. A further class of compounds which is suitable for reducing themelting point in the phases according to the invention is that of theformula ##STR11## in which R⁴ and R⁵ have the preferred meaningindicated for IVc, IVh and IVi.

Suitable further components having negative dielectric anisotropy arefurthermore compounds containing the structural element A, B or C.##STR12##

Preferred compounds of this type conform to the formulae Va, Vb and Vc:##STR13## R' and R" are each preferably straight-chain alkyl or alkoxygroups, in each case having 2 to 10 carbon atoms. Q¹ and Q² are each1,4-phenylene, trans-1,4-cyclohexylene, 4,4'-biphenylyl,4-(trans-4-cyclohexyl)phenyl, trans,trans-4,4'-bicyclohexyl or one ofthe groups Q¹ and Q² is alternatively a single bond.

Q³ and Q⁴ are each 1,4-phenylene, 4,4'-biphenylyl ortrans-1,4-cyclohexylene. One of the groups Q³ and Q⁴ can alternativelybe 1,4-phenylene in which at least one CH group has been replaced by N.R"' is an optically active radical containing an asymmetric carbon atomof the structure ##STR14## Particularly preferred compounds of theformula Vc are those of the formula Vc' ##STR15## in which A is1,4-phenylene or trans-1,4-cyclohexylene, and n is 0 or 1.

Particular preference is given to ferroelectric liquid-crystallinephases which contain at least one achiral halopyridine of the formulaI1, at least one achiral phenylpyrimidine of the formula Vc' in whichR"' is an alkyl or alkoxy radical having up to 18 carbon atoms, as thebase material having a broad S_(c) phase and at least one chiralhalopyridine of the formula I as optically active dope. Preference isfurthermore given to ferroelectric liquid-crystalline phases which, inaddition to said compounds of the formula I1, Vc' and I, contain atleast one phenylpyridine of the formula Vd and/or a 2-fluoro- (L=H) or2,3-difluorophenylpyrimidine of the formula Ve (L=F) and/or aphenylpyrimidine of the formula Vf ##STR16##

The achiral halopyridines of the formula IA ##STR17## in which alkyl isin each case, independently of one another, alkyl having 1 to 18 carbonatoms,

X and Y are each, independently of one another, CH or N,

Q⁴ is ##STR18## or a single bond, and Q⁵ is ##STR19## or a single bond,are particularly suitable as constituents of smectic base materials.

The compounds of the formula IA include the achiral preferred [sic]bicyclic and tricyclic materials of the formulae IA1 to IA18 listedbelow ##STR20## where, in the compounds IA9 to IA18, Q⁴ is --O-- or asingle bond and Q⁵ is --O--, --O--CO-- or a single bond, and

X and Y are each, independently of one another, N or CH, preferably CH.

Furthermore preferred compounds of the formula I are those of theformulae [sic] IB ##STR21## in which alkyl is in each case,independently of one another, alkyl having 1 to 18 carbon atoms,

Q⁶ is --O--, --CO--O--, --O--CO-- or a single bond,

Q⁷ is --CO-- or --CH₂ --, ##STR22## is in each case, independently ofone another, ##STR23## where o is 0, 1 or 2, and m is 1 or 2.

These compounds of the formula IB are particularly suitable asconstituents of nematic mixtures.

The compounds of the formula IB include the preferred [sic] bicyclic andtricyclic materials of the formula [sic] IB1 to IB8 listed below:##STR24##

Furthermore preferred compounds of the formula I are those in which theradical Z¹ or Z² linked to the 6-halopyridine-2,5-diyl is a --C.tbd.C--group and A¹ and/or A² is 1,4-phenylene, which is optionally substitutedby 1 or 2 fluorine atoms, or 1,4-cyclohexylene. These compounds includethe preferred compounds of the formulae IT1 to IT6, ##STR25## in which ois 0, 1 or 2.

In the preferred compounds of the formula [sic] IT1 to IT6, L ispreferably F and o is 0; particular preference is given to the compoundsof the formula [sic] IT2 and IT4 in which L is F and R¹ is alkoxy having1 to carbon atoms.

The compound [sic] of the formula [sic] IT1 to IT6 are particularlysuitable as components of nematic phases for ECB or STN displays.

In the formulae above and below, R¹ and R² preferably have 1-13 carbonatoms, in particular 3-12 carbon atoms. Compounds of the formula I inwhich R¹ and R² have 1-7 carbon atoms, preferably 1-5 carbon atoms, areparticularly suitable for liquid-crystalline phases for display elementsbased on the ECB effect. By contrast, compounds of the formula I inwhich R¹ and R² have 6-15 carbon atoms, preferably 6-12 carbon atoms,are suitable for liquid-crystalline phases having ferroelectricproperties. It is also possible for one or two CH₂ groups in R¹ and R²to have been replaced. It is preferred that only one CH₂ group has beenreplaced by --O--, --S--, --CO--O-- or --O--CO--, in particular by--O--.

Preference is furthermore given to compounds in which 2, 3 or [lacuna]non-adjacent CH₂ groups have been replaced by --O--.

In the formulae above and below, R¹ and R² are preferably alkyl, alkoxyor another oxaalkyl group.

If R₁ [sic] and R₂ [sic] are alkyl radicals in which one ("alkoxy" or"oxaalkyl") or two ("alkoxyalkoxy" or "dioxaalkyl") non-adjacent CH₂groups may also have been replaced by O atoms, they may bestraight-chain or branched. They are preferably straight-chain, have 2,3, 4, 5, 6 or 7 carbon atoms and accordingly are preferably ethyl,propyl, butyl, pentyl, hexyl, heptyl, ethoxy, propoxy, butoxy, pentoxy,hexoxy or heptoxy, furthermore methyl, octyl, nonyl, decyl, undecyl,dodecyl, tridecyl, tetradecyl, pentadecyl, methoxy, octoxy, nonoxy,decoxy, undecoxy, tridecoxy or tetradecoxy.

Oxalkyl is preferably straight-chain 2-oxypropyl [sic] (=methoxymethyl),2- (=ethoxymethyl) or 3-oxabutyl (=2 methoxyethyl), 2-, 3- or4-oxapentyl, 2-, 3-, 4- or 5-oxahexyl, 2-, 3-, 4-, 5- or 6-oxaheptyl,2-, 3-, 4-, 5-, 6- or 7-oxaoctyl, 2-, 3-, 4-, 5-, 6-, 7- or 8-oxanonyl,2-, 3-, 4-, 5-, 6-, 7-, 8- or 9-oxadecyl, 1,3-dioxabutyl(=methoxymethoxy), 1,3-, 1,4- or 2,4-dioxapentyl, 1,3-, 1,4-, 1,5-,2,4-, 2,5- or 3,5-dioxahexyl, or 1,3-, 1,4-, 1,5-, 1,6-, 2,4-, 2,5-,2,6-, 3,5-, 3,6- or 4,6-dioxaheptyl.

The compounds of the formula I which contain one or two dioxa- ortrioxaalkyl group [sic] are particularly suitable as complexing podandsfor reducing the concentration of free ions in liquid-crystallinemixtures, in particular for ferroelectric displays. In particular, thesecompounds can be used to suppress so-called "ghost images" (cf., forexample, J. Dijon et al., SID conference, San Diego 1988, pages 2-249).

If R¹ and R² are an alkyl radical in which one CH₂ group has beenreplaced by --S--, this may be straight-chain and [sic] branched. Thisthiaalkyl radical is preferably straight-chain and is 2-thiapropyl, 2-or 3-thiabutyl, 2-, 3- or 4-thiapentyl, 2-, 3-, 4- or 5-thiahexyl, 2-,3-, 4-, 5- or 6-thiaheptyl, 2-, 3-, 4-, 5-, 6- or 7-thiaoctyl, 2-, 3-,4-, 5-, 6-, 7- or 8-thianonyl or 2-, 3-, 4-, 5-, 6-, 7-, 8- or9-thiadecyl.

Particularly preferred alkyl radicals R¹ or R² are those in which theCH₂ group adjacent to the A¹ A² and/or A³ group has been replaced by--S-- and is thus preferably methylthio, ethylthio, propylthio,butylthio, pentylthio, hexylthio, heptylthio, octylthio, nonylthio ordecylthio.

If R¹ and R² are an alkenyl radical, this may be straight-chain orbranched. It is preferably straight-chain and has 2 to 10 carbon atoms.It is accordingly particularly vinyl, prop-1- or prop-2-enyl, but-1-, 2-or but-3-enyl, pent-1-, 2-, 3- or pent-4-entyl [sic], hex-1-, 2-, 3-, 4-or hex-5-enyl, hept-1-, 2-, 3-, 4-, 5- or hept-6-enyl, oct-1-, 2-, 3-,4-, 5-, 6- or oct-7-enyl, non-1-, 2-, 3-, 4-, 5-, 6-, 7-or non-8-enyl,or dec-1-, 2-, 3-, 4-, 5-, 6-, 7-, 8-or dec-9-enyl.

If R¹ and R² are an alkyl radical in which one CH₂ group has beenreplaced by --O--CO or --CO--O--, this may be straight-chain orbranched. It is preferably straight-chain and has 2 to 6 carbon atoms.It is accordingly particularly acetyloxy, propionyloxy, butyryloxy,pentanoyloxy, hexanoyloxy, acetyloxymethyl, propionyloxymethyl,butyryloxymethyl, pentaoyloxymethyl [sic], 2-acetyloxyethyl,2-propionyloxyethyl, 2-butyryloxyethyl, 3-acetyloxypropyl,3-propionyloxypropyl, 4-acetyloxybutyl, methoxycarbonyl, ethoxycarbonyl,propoxycarbonyl, butoxycarbonyl, pentoxycarbonyl, methoxycarbonylmethyl,ethoxycarbonylmethyl, propoxycarbonylmethyl, butoxycarbonylmethyl,2-(methoxycarbonyl)ethyl, 2-(ethoxycarbonyl)ethyl,2-(propoxycarbonyl)ethyl, 3-(methoxycarbonyl)propyl,3-(ethoxycarbonyl)propyl or 4-(methoxycarbonyl)butyl.

Compounds of the formula I containing branched wing groups R¹ and/or R²may occasionally be of importance due to better solubility in theconventional liquid-crystalline base materials, but in particular aschiral dopes if they are optically active. Smectic compounds of thistype are suitable as components of ferroelectric materials.

Branched groups of this type generally contain not more than one chainbranch. Preferred branched radicals R¹ and/or R² are isopropyl, 2-butyl(=1-methylpropyl), isobutyl (=2-methylpropyl), 2-methylbutyl, isopentyl(=3-methylbutyl), 2-methylpentyl, 3-methylpentyl, 2-ethylhexyl,2-propylpentyl, isopropoxy, 2-methylpropoxy, 2-methylbutoxy,3-methylbutoxy, 2-methylpentoxy, 3-methylpentoxy, 2-ethylhexoxy,1-methylhexoxy, 1-methylheptoxy, 2-oxa-3-methylbutyl,3-oxa-4-methylpentyl, 4-methylhexyl, 2-nonyl, 2-decyl, 2-dodecyl,6-methyloctoxy, 6-methyloctanoyloxy, 5-methylheptyloxycarbonyl,2-methylbutyryloxy, 3-methylvaleryloxy, 4-methylhexanoyloxy,2-chloropropionyloxy, 2-chloro-3-methylbutyryloxy,2-chloro-4-methylvaleryloxy, 2-chloro-3-methylvaleryloxy,2-methyl-3-oxapentyl and 2-methyl-3-oxahexyl.

Particular preference is given to the compounds of the formula I inwhich L=F, m=0 and R¹ and R² are each, independently of one another,alkyl having up to 15 carbon atoms in which, in addition, one or moreCH₂ groups may have been replaced by 0 [sic] atoms. R¹ is preferablyalkoxy having up to 12 carbon atoms.

The chiral halopyridines of the formula I in which one of the radicalsR¹ and R² is a group of the formula III are highly suitable as dopes forinducing ferroelectricity in a smectic base material. They aredistinguished, in particular, by high spontaneous polarization.

Furthermore, they do not destabilize the smectic phase of these basematerials. The radical of the formula III is referred to below as R*.

The chiral compounds of the formula I accordingly include the bicycliccompounds of the subformulae Ic1 to Ic4 ##STR26## and the tricycliccompounds of the formulae Ic5 to Ic10: ##STR27## in which R¹, R², L Z²and Z^(2'), are as defined above, and R^(2'), in the case where L=Cl,has the same meaning as R², but in the case where L=F, where halogen,--CF₃, --OCF₃, --OCHF₂ and alkoxy are excluded. In the case where L=Cl,Z^(2') has the same meaning as Z² and in the case where L=F, Z^(2') is--CH₂ CH₂ --, --C--C--, --CH₂ O--, --CO--O-- or a single bond.

Particularly preferred compounds are those of the subformulae Ic1 to Ic4in which R¹ and R² are each alkyl or alkoxy having 1 to 12 carbon atomsor R^(2') is alkyl, oxaalkyl or dioxaalkyl having 1 to 12 carbon atoms,and A¹ or A² is 1,4-phenylene in which, in addition, 1 or 2 CH groupsmay have been replaced by N.

Of these, particular preference is given to the chiral compounds of theformula I in which R* is a group of the formula IIIa ##STR28## in whicho and Q³ are as defined above, Q⁴ is --O--, --CO--O-- or a single bond,r is 1 or 2, and p is 0 or 1.

The chiral radicals of the formula IIIa accordingly include the chiralmonofluoroalkyl, monofluoroaoxaalkyl [sic] andalkanoyloxymonofluoroalkyl group R_(f) * of the formulae IIIa1 to IIIa6:##STR29##

Of the chiral monofluoro groups R_(f) * of the formulae IIIa1 to IIIa6,particular preference is given to those of the formulae IIIa1, IIIa3 andIIIa5, in particular those in which r is 2.

Preference is furthermore given to the chiral compounds of the formula Iin which R* is a group of the formula IIIb ##STR30## in which o, Q³ andQ⁴ are as defined above, r is 1 or 2, and p is 0 or 1.

Preference is furthermore given to the compounds of the formula I inwhich R* is a group of the formula IIIc ##STR31## in which Q³, Q⁴, r andp are as defined above.

The compounds of the formula I are prepared by methods which are knownper se, as described in the literature (for example in the standardworks such as Houben-Weyl, Methoden der Organischen Chemie,Georg-Thieme-Verlag, Stuttgart).

If desired, the starting materials can also be formed in situ by notisolating them from the reaction mixture, but instead immediatelyfurther converting them into the compounds of the formula I.

The compounds according to the invention can easily be prepared inaccordance with the reaction schemes (1-9 [sic]) below. ##STR32##

The benzyl group can be removed hydrogenolytically, and the hydroxylgroup can then be re-etherified or re-esterified by known methods.

The chiral compounds according to the invention can be prepared inaccordance with the reaction schemes (I to VII) below.

Thus, compounds of the formula I in which the chiral radical R* has theformula IIIa where s=2 and p=1 can be prepared by preparing suitableprecursors from optically active malic acid in accordance with thereaction scheme I: ##STR33##

The synthesis has been described by Mori et al. as far as this stage (KoMori, T. Takigawa and T. Matsuo, Tetrahedron 35, 933-944 (1979)).

Meyers and Lawson later found that the chemical purity of the acetonideobtained by this route is only about 90% (A. I. Meyers and J. P. Lawson,THL 23 4883-4886 (1982)).

This notwithstanding, the free alcohol group of the acetonide can beetherified by one of the conventional methods (for example C. A. Brownand D. Barton, Synthesis (1974) 434 or B. R. Jursic, Tetrahedron 44,6677-6680 (1988)).

The benzyl ether (K. Isaac and P. Kocienski, J. Chem. Soc., Chem.Commun. (1982) 460-462) is a particularly suitable protecting groupsince it can later easily be removed hydrogenolytically. Afteretherification, the isopropylidene ketal is hydrolyzed under standardconditions to give the 1,2-diol, and this is then converted into thecorresponding epoxide under the reaction conditions of Di Fabio andMisiti (R. Di Fabio and D. Misiti, Gazetta Chimica Italiana 118, 209-210(1988)).

Treatment of the acetonide with HBr/glacial acetic acid and subsequentreaction of the bromooxalkyl acetates obtained in this way with Kpentoxide likewise gives the desired epoxides in accordance with SchemeII, according to the paper by U. Schmidt et al. (U. Schmidt, J.Tabiersky, F. Bartowiak and J. Wild, Angew. Chem. 92, 201-202 (1980)).##STR34##

Reaction of the epoxide with organometallic compounds in accordance withScheme IIa, preferably with Grignard compounds, with ring opening on theless-substituted carbon atom of the epoxide, gives, with highselectivity, the corresponding alcohol, which is fluorinated using DASTunder standard conditions; hydrogenolysis gives the chiral alcohol,which is etherified using 5-hydroxypyridines. ##STR35##

If the expoxide is opened using pyridine/HF (N. Mongelli, F. Animati etal., Synthesis 310 (1988)), the corresponding fluoroalcohol is obtained,which can subsequently be converted into the corresponding tosylate.Such tosylates are particularly suitable for alkylation of phenols and5-hydroxypyridines in accordance with Scheme III or Scheme IV. ##STR36##

As the above reaction schemes show, the epoxide can also be reacteddirectly with phenols. The epoxide is opened in high selectivity at theless-substituted carbon atom to give the chiral secondary alcohol, whichis then finally converted into the compounds according to the inventionusing DAST with inversion. Regarding the conventional reactions ofalcohols with DAST, see: M. Hudlicky, Organic Reactions 35 513-637(1987).

The compounds according to the invention where Q³ =--O--CO-- can beprepared from the corresponding benzyl ethers by hydrogenolysis andsubsequent esterification. The synthesis scheme V below describes thepreparation: ##STR37##

The compounds where Q³ =--O--CO-- are furthermore obtained by oxidationof the corresponding fluoroalcohols and subsequent esterification usingmesogenic phenols in accordance with Scheme VI: ##STR38##

If racemization occurs during oxidation, the optically active fluoroacids can be isolated by separating the racemate by the method ofHelmchen (Angew. Chem. 91, 65 (1979)).

CH-acidic compounds likewise open the epoxide in the presence ofsuitable bases to give the optically active secondary alcohol, which isthen fluorinated using DAST with inversion. Preferred reaction routesare given in Scheme VII below. ##STR39##

The compounds containing chiral radicals R* of the formula IIIa in whichp is 1, s is 1 and Q³ is O can be prepared analogously using the knownepoxides of the formula ##STR40## or the fluoroalcohols of the formula##STR41## obtainable therefrom by conventional methods.

The compounds of the formula I are also suitable as components ofnematic liquid-crystalline phases, in particular for ECB displays, butalso for preventing reverse twist.

These liquid-crystalline phases according to the invention comprise 2 to25, preferably 3 to 15 components, including at least one compound ofthe formula I. The other constituents are preferably selected fromnematic or nematogenic substances, in particular known substances, fromthe classes comprising the azoxybenzenes, benzylidene anilines,biphenyls, terphenyls, phenyl or cyclohexyl benzoates, phenyl orcyclohexyl cyclohexanecarboxylates, phenylcyclohexanes,cyclohexylbiphenyls, cyclohexylcyclohexanes, cyclohexylnaphthalenes,1,4-bis-cyclohexylbenzenes, 4,4'-bis-cyclohexylbiphenyls, phenyl- orcyclohexylpyrimidines, phenyl- or cyclohexylpyridazines and N-oxidesthereof, phenyl- or cyclohexyldioxanes, phenyl- orcyclohexyl-1,3-dithianes, 1,2-diphenylethanes, 1,2-dicyclohexylethanes,1-phenyl-2-cyclohexylethanes, optionally halogenated stilbenes, benzylphenyl ethers, tolans and substituted cinnamic acids.

The most important compounds which are suitable as components ofliquid-crystalline phases of this type can be characterized by theformula I"

    R'--L--G--E--R"                                            I"

in which L and E are each a carbocyclic or heterocyclic ring system fromthe group comprising 1,4-disubstituted benzene and cyclohexane rings,4,4'-disubstituted biphenyl, phenylcyclohexane and cyclohexylcyclohexanesystems, 2,5-disubstituted pyrimidine and 1,3-dioxane rings,2,6-disubstituted naphthalene, di- and tetrahydronaphthalene,quinazoline and tetrahydroquinazoline,

G is ##STR42## Y is halogen, preferably chlorine, or --CN, and R' and R"are alkyl, alkoxy, alkanoyloxy, alkoxycarbonyl or alkoxycarbonyloxyhaving up to 18, preferably up to 8, carbon atoms, or one of theseradicals is alternatively CN, NC, NO₂, CF₃, F, Cl or Br.

In most of these compounds, R' and R" are different from one another,one of these radicals usually being an alkyl or alkoxy group. However,other variants of the proposed substituents are also common. Many suchsubstances or mixtures thereof are commercially available. All thesesubstances can be prepared by methods which are known from theliterature.

The phases according to the invention contain about 0.1 to 99%,preferably 10 to 95%, of one or more compounds of the formula I.Additionally preferred liquid-crystalline phases according to theinvention are those which contain 0.1-40%, particularly 0.5-30%, of oneor more compounds of the formula I. The phases according to theinvention are prepared in a conventional manner. In general, thecomponents are dissolved in one another, expediently at elevatedtemperature. By means of suitable additives, the liquid-crystallinephases according to the invention can be modified in a manner such thatthey can be used in all types of liquid-crystal display elements whichhave been disclosed hitherto.

Additives of this type are known to those skilled in the art and aredescribed in detail in the literature. For example, conductive salts,preferably ethyldimethyl-dodecylammonium 4-hexyloxybenzoate,tetrabutylammonium tetraphenylborate or complex salts of crown ethers(cf., for example, I. Haller et al., Mol. Cryst. Liq. Cryst. Volume 24,pages 249-258 (1973)) can be added in order to improve the conductivity,pleochroic dyes can be added to produce colored guest-host systems orsubstances can be added to modify the dielectric anisotropy, theviscosity and/or the orientation of the nematic phases.

Substances of this type are described, for example, in DE-A 2,209,127,2,240,864, 2,321,632, 2,338,281, 2,450,088, 2,637,430, 2,853,728 and2,902,177.

The mixtures described are particularly suitable as components ofliquid-crystal switching and display devices, in particular for theSSFLC cell described at the outset. This applies in particular to theso-called "virgin liquid-crystal structure", in which the smectic layersare at an angle to one another (chevron structures") and to the"bookshelf" or "quasi-bookshelf" geometry, in which the smectic layersare perpendicular or virtually perpendicular (see Y. Sato et al., Jap.J. Appl. Phys. Vol. 28, 483 (1989)).

In addition, the nematic mixtures containing compounds of the formula I,due to their broad nematic phase ranges and their high negativedielectric anisotropy, are particularly suitable as components of morehighly twisted liquid-crystal displays, such as STN (for example GB2,123,163) or OMI (for example M. Schadt, F. Leenhouts, Appl. Phys.Lett. 50, 236 (1987)), in which very high gradients of thecharacteristic line can be achieved (for example WO 89/08691).

In particular, the compounds of the formula IT are highly suitable ascomponents of ECB displays (for example M. Schiekel, K. Fahrenschon,Appl. Phys. Lett. 19 391 (1971)), which [sic] they have very high valuesof the optical anisotropy (Δn≧0.22) and comparatively high-negativevalues of the dielectric anisotropy (Δε≦-5.0).

The examples below are intended to illustrate the invention withoutrepresenting a limitation. m.p.=melting point, c.p.=clearing point.Above and below, percentages are percent by weight; all temperatures aregiven in degrees Celsius. "Customary work-up" means that water is added,the mixture is extracted with methylene chloride, the organic phase isseparated off, dried and evaporated, and the product is purified bycrystallization and/or chromatography.

In addition:

    ______________________________________                                        In addition:                                                                  ______________________________________                                        C                denotes crystalline                                          N                denotes nematic                                              S                denotes smectic                                              I                denotes isotropic.                                           ______________________________________                                    

The numbers between these symbols indicate the respectivephase-transition temperature in °C. The following abbreviations are alsoused:

    ______________________________________                                        n-BuLi n-butyllithium                                                         DAST   diethylaminosulfur trifluoride                                         DCC    dicyclohexylcarbodiimide                                               DEAD   diethyl azodicarboxylate                                               DMF    dimethylformamide                                                      LDA    Lithian [sic] diisopropylamide (prepared from                                 equimolar amounts of n-BuLi and diisopropylamine)                      THP    tetrahydrofuran                                                        TP     triphenylphosphine                                                     TPP    tetrakis (triphenylphosphine)palladium (0)                             ______________________________________                                    

EXAMPLE 1 1A1-(trans-4-Pentylcyclohexyl)-4-(6-chloropyridin-2-yl)benzene

A mixture of 0.23 mol of 4-(trans-4-pentylcyclohexyl)phenylboric acid,0.23 mol of 2,6-dichloropyridine, 5 mmol of TPP, 230 ml of a 2 molarsodium carbonate solution and 400 ml of toluene are heated at the boilfor 1 hour. The reaction mixture is cooled, and 200 ml of hexane and 200ml of water are added. The organic phase is separated off, the solventis removed by evaporation, and the pure product is obtained bychromatography on silica gel using toluene/hexane 3:1 as the eluent.

1B 2-[4-(trans-4-Pentylcylcohexyl)phenyl]-5-hydroxy-2-chloropyridine[sic]

A mixture of 327.5 mmol of 1A and 10 ml of THF is added dropwise at -65°C. to a mixture of 42 mmol of LDA and 40 ml of THF, and the mixture isstirred for 15 minutes. 38.1 mmol of trimethyl borate are subsequentlyadded dropwise. The mixture is warmed to -20° C., and 3.5 ml of glacialacetic acid and 5 ml of water are added. The mixture is warmed to roomtemperature, and 9 ml of H₂ O₂ (30%) are added. The mixture is stirredfor 20 hours and distributed between 200 ml of water and 200 ml of THF.The organic phase is separated off and shaken with a dilute ammoniumiodide solution in order to remove peroxides.

After removal of the solvent by distillation, the product is processedfurther without purification.

IC 2-[4-(trans-4-Pentylcyclohexyl)phenyl]-5-ethoxy-2 -chloropyridine

A mixture of 33.5 mmol of 1B, 34.2 mmol of TP, 34.2 mmol of DEAD, 34.2mmol of ethanol and 200 m [sic] of THF is stirred for 20 hours at roomtemperature. Customary work-up and chromatography give the pure product,C 116.0 N 150.4 I, Δε=-5.3.

The following are prepared analogously

    ______________________________________                                         ##STR43##                                                                     R.sup.1    n                                                                                   ##STR44##         R.sup.2                                   ______________________________________                                        C.sub.3 H.sub.7                                                                          0     --                C.sub.2 H.sub.5                            C.sub.5 H.sub.11                                                                         0     --                C.sub.2 H.sub.5                            C.sub.8 H.sub.17                                                                         0     --                C.sub.2 H.sub.5                            C.sub.8 H.sub.17                                                                         0     --                C.sub.8 H.sub.17                           C.sub.3 H.sub.7                                                                          1                                                                                    ##STR45##        C.sub.8 H.sub.17                           C.sub.5 H.sub.11                                                                         1                                                                                    ##STR46##        C.sub.8 H.sub.17                           C.sub.8 H.sub.17                                                                         1                                                                                    ##STR47##        C.sub.8 H.sub.17                           C.sub.3 H.sub.7                                                                          1                                                                                    ##STR48##        C.sub.3 H.sub.7                            C.sub.5 H.sub.11                                                                         1                                                                                    ##STR49##        C.sub.3 H.sub.7                            C.sub.8 H.sub.17                                                                         1                                                                                    ##STR50##        C.sub.8 H.sub.17                           ______________________________________                                    

EXAMPLE 2 2A 2-Fluoro-6-phenylsulfonylpyridine

A mixture of 0.4 mol of 2,6-difluoropyridine, 0.6 mol of thiophenol, 0.6mol of NaOH, 450 ml of water, 180 ml of toluene and 3.5 g oftetrabutylammonium bromide is refluxed for hours.

The mixture is cooled to room temperature, the organic phase isseparated off, and the aqueous phase is extracted with hexane. Thecombined organic phases are washed with a saturated sodium chloridesolution and dried using sodium sulfate. The solvent is evaporated, 22 gof a 35% H₂ O₂ solution and 50 ml of glacial acetic acid are added tothe residue, and the mixture is heated at the boil for 18 hours.Cooling, customary work-up and chromatography on silica gel usingdichloromethane gives the pure product.

2B 2-(4-Octyloxyphenyl)-6-fluoropyridine

A mixture of 0.06 mol of 4-octyloxyphenylmagnesium bromide (preparedfrom 0.06 mol of 4-octyloxybromobenzene and 0.06 mol of4-octyloxybromobenzene and 0.06 mol of magnesium) and 25 ml of THF isadded at 30°-40° C. to a mixture of 0.05 mol of 2A and 15 ml of THF. Themixture is stirred at room temperature for 2 hours and subjected tocustomary work-up. The unpurified product is processed further inaccordance with 2C.

2C 2-(4-Octyloxyphenyl)-5-hydroxy-6-fluoropyridine

The product is obtained analogously to Example 1B [lacuna] 0.05 mol of2B, 0.08 mol of LDA, 0.08 mol of trimethyl borate, and 0.16 mol of H₂ O₂(in the form of a 30% solution).

2D 2-(Octyloxyphenyl)-5-octyloxy-6-fluoropyridine

The product, C 81 S_(c) 89 I, Δε=-4.3, Δn=0.161, is obtained analogouslyto Example 1C from 20 mmol of 2C, 20 mmol of octanol, 20 mmol of TP and20 mmol of DEAD.

The following are prepared analogously:

    ______________________________________                                         ##STR51##                                                                     R.sup.1                                                                              n                                                                                   ##STR52##                                                                                 ##STR53##                                                                               R.sup.2                                   ______________________________________                                        C.sub.8 H.sub.17 O                                                                   0     --                                                                                         ##STR54##                                                                              OC.sub.10 H.sub.21 C 86 S.sub.C 90.0                                          I                                          C.sub.8 H.sub.17                                                                     0     --                                                                                         ##STR55##                                                                              OC.sub.8 H.sub.17 C 38 S.sub.C 57 I,                                          Δε = -4.63                   C.sub.10 H.sub.21 O                                                                  0     --                                                                                         ##STR56##                                                                              OC.sub.8 H.sub.17                          C.sub.3 H.sub.7                                                                      0     --                                                                                         ##STR57##                                                                              OC.sub.2 H.sub.5 C 82                                                         I, Δε -7                     C.sub.5 H.sub.11                                                                     0     --                    OC.sub.2 H.sub.5 C 64 I,                                                      Δε -6.2                      C.sub.5 H.sub.11                                                                     0     --                                                                                         ##STR58##                                                                              OC.sub.4 H.sub.9                           C.sub.3 H.sub.7                                                                      0     --                                                                                         ##STR59##                                                                              OC.sub.4 H.sub.9 C 59                                                         I, Δε -6                     C.sub.5 H.sub.11                                                                     0     --                                                                                         ##STR60##                                                                              OCH.sub.3 C 69 I, Δε                                            -6.2                                       C.sub.5 H.sub.11                                                                     1                                                                                    ##STR61##                                                                                 ##STR62##                                                                              OC.sub.2 H.sub.5 C 112 S.sub.A (105) N                                        189.7 I, Δε = -5.3                                              Δn = 0.211                           C.sub.8 H.sub.17                                                                     1                                                                                    ##STR63##                                                                                 ##STR64##                                                                              OC.sub.8 H.sub.17                          C.sub.5 H.sub.11                                                                     1                                                                                    ##STR65##                                                                                 ##STR66##                                                                              C.sub.3 H.sub.7 C 60 S.sub.G 86                                               S.sub.F 91 S.sub.A 124 N 150 I,                                               Δε = -3.05                   C.sub.3 H.sub.7                                                                      1                                                                                    ##STR67##                                                                                 ##STR68##                                                                              OC.sub.2 H.sub.5 C 81 S.sub.B 98 N                                            178.5 I Δε = -6.13           C.sub.2 H.sub.5 O                                                                    0     --                                                                                         ##STR69##                                                                              C.sub.3 H.sub.7                            C.sub.2 H.sub.5 O                                                                    0     --                                                                                         ##STR70##                                                                              C.sub.5 H.sub.11                           C.sub.8 H.sub.17 O                                                                   0     --                                                                                         ##STR71##                                                                              C.sub.10 H.sub.21                          C.sub.10 H.sub.21 O                                                                  0     --                                                                                         ##STR72##                                                                              C.sub.10 H.sub.21                          C.sub.10 H.sub.21                                                                    0     --                                                                                         ##STR73##                                                                              C.sub.10 H.sub.21                          ______________________________________                                    

EXAMPLE 3

A solution of 0.1 mol of2-(octylphenyl)-5-(2-hydroxy-5-oxaoctyloxy)-6-fluoropyridine (preparedby heating optically active 1,2-epoxy-5-oxaoctane, obtainable from malicacid, with 2C in the presence of dry potassium carbonate and methylethyl ketone as solvent) in methylene chloride is cooled to -40° C., and0.11 mol of DAST is added dropwise thereto with exclusion of moisture.The reaction mixture is subsequently stirred for 12 hours with slowwarming to room temperature. The reaction mixture is then hydrolyzedwith ice cooling and washed with dilute sodium hydroxide solution andseveral times with water. The mixture is dried over magnesium sulfate,the solvent is removed on a rotary evaporator, and the crude product ispurified by chromatography and by crystallization, giving opticallyactive 2-(4-octyloxyphenyl)-5-(2-fluoro-5-oxaoctyloxy)-6-fluoropyridine.

The following compounds of the formula I1 are prepared analogously:

    ______________________________________                                         ##STR74##                     I1                                                      m   n                                                                ______________________________________                                                 2   7                                                                         4   7                                                                         5   7                                                                         2   8                                                                         3   8                                                                         4   8                                                                         5   8                                                                         3   9                                                                         3   10                                                               ______________________________________                                    

EXAMPLE 4

4-(6-Fluoro-5-octyloxypyridin-2-yl)-1-(2-fluoro-5-oxyoctyl)benzene [sic]is obtained analogously to Example 3 from 0.1 mol of4-(6-fluoro-5-octyloxypyridin-2-yl)phenol (prepared as in Example 2 byhydrolysis of 4-(6-fluoro-5-octyloxypyridin-2-yl)-1-benzyloxybenzene)and 0.1 mol of optically active 1,2-epoxy-5-octane.

The following compounds of the formula I1 are prepared analogously:

    ______________________________________                                         ##STR75##                     I1                                             R.sup.1      n               X                                                ______________________________________                                        C.sub.7 H.sub.15 O                                                                         3               O                                                C.sub.9 H.sub.19 O                                                                         3               O                                                C.sub.11 H.sub.23 O                                                                        3               O                                                C.sub.8 H.sub.17                                                                           3               CH.sub.2                                         ______________________________________                                    

EXAMPLE 4 a

4-(6-Fluoro-5-(2-fluoro-5-oxyoctyl)pyridin-2-yl)-1-octylbenzene [sic]0.1 mol of 2 C (prepared as in Example 1) and 0.1 mol of opticallyactive 2-fluoro-1-octanol are dissolved in 200 ml of THF, and 0.1 m[sic] of triphenylphosphine and 0.1 m [sic] of DEAD are added at RT. Themixture is stirred at RT for a further 2 hours and worked up byextraction: [lacuna]

    ______________________________________                                         ##STR76##                                                                    R.sup.1      n           R.sup.1 n                                            ______________________________________                                        C.sub.8 H.sub.17                                                                           5 C 80 I                                                         C.sub.7 H.sub.15 O                                                                         5           C.sub.7 H.sub.15                                                                      5                                            C.sub.8 H.sub.17 O                                                                         5           C.sub.8 H.sub.17                                                                      5                                            C.sub.9 H.sub.19 O                                                                         5           C.sub.9 H.sub.19                                                                      5                                            C.sub.10 H.sub.21 O                                                                        5           C.sub.10 H.sub.21                                                                     5                                            ______________________________________                                    

EXAMPLE 5

0.17 mol of DEAD dissolved in THF is added to a solution of 0.15 mol of2C (prepared as in Example 1), 0.17 mol of L(-)-ethyl lactate and 0.15mol of triphenylphosphine in 400 ml of THF. A reaction temperature of50° C. should not be exceeded. The mixture is stirred at 50° C. for 1hour and then at room temperature overnight. The solvent is then removedby distillation, the residue is dissolved in hot toluene, and thesolution is then allowed to cool slowly. The precipitatedtriphenylphosphine oxide is removed by suction filtration, the filtrateis evaporated, the residue is purified by chromatography, giving ethyl2-[2-(4-octyloxyphenyl)-6-fluoropyridin-5-yl]propionate.

EXAMPLE 6

Optically active benzyl lactate is etherified using 2C (prepared as inExample 1) by means of diethyl azodicarboxylate(DEAD)/triphenylphosphine, and the benzyl group is subsequently removedhydrogenolytically. The acid obtained in this way is converted as usualinto the nitrile (oxalyl chloride, ammonia, thionyl chloride), givingoptically active2-(4-octyloxyphenyl)-5-(1-cyanoethoxy)-6-fluoropyridine.

EXAMPLE 7

A solution of 0.1 mol of DCC in methylene chloride is added at 0° C. toa mixture of 0.1 mol of 2C 0.1 mol of optically active2-chloro-3-methylbutyric acid (prepared from valine) and a catalyticamount of 4-(N,N-dimethylamino)pyridine in 250 ml of methylene chloride.The mixture is subsequently stirred at room temperature for 12 hours,the precipitate is then filtered off with suction, and the filtrate issubjected to customary work-up, giving[2-(4-octyloxyphenyl)-6-fluoropyridine-5-yl ester] [sic].

EXAMPLE 8

0.2 mmol of bis(triphenylphosphine)palladium(II) chloride and 0.1 mmolof copper(I) iodide are added at room temperature to a mixture of 0.01mol of 5-ethoxy-2-iodo-6-fluoropyridine (prepared from 2-fluoropyridineby reaction with lithium diisopropylamide, triethyl borate and hydrogenperoxide, iodination of the resultant 3-hydroxy-2-fluoropyridine, andsubsequent etherification using iodoethane/potassium carbonate), 0.01mol of 4-(trans-4-propylcyclohexyl)phenylacetylene (preparable, forexample, by the method of Smith, Hoehn, Am. Soc. 63 (1941) 1175) and 40ml of triethylamine, and the mixture is stirred for 12 hours. Thereaction can be monitored by means of thin-layer chromatography. Whenthe reaction is complete, the suspension is filtered, and the filtrateis evaporated. Purification by chromatography and/or crystallizationgives1-(5-ethoxy-6-fluoropyridin-2-yl)-1-[4-(trans-4-propylcyclohexyl)phenyl]acetylene.

The following are [sic] prepared analogously:

1-(5-Ethoxy-6-fluoropyridin-2-yl)-1-(4-pentylphenyl)acetylene, C 69 N(54.1) I Δε=-7.83, Δn =0.252.

EXAMPLE 9

A mixture of 0.1 mol of 2C, 0.1 mol of 1-bromoethoxyethane, 0.1 mol ofsodium hydroxide, 20 ml of ethanol and 50 ml of water is heated at theboil for 2 hours. The mixture is cooled, dilute hydrochloric acid isadded, and the aqueous phase is extracted with dichloromethane. Thesolvent is removed under reduced pressure, and the residue is purifiedby column chromatography, giving the product2-(4-octyloxyphenyl)-5-(2-ethoxyethoxy)-6-fluoropyridine.

The following are prepared analogously:

    ______________________________________                                         ##STR77##                                                                    R.sup.1           R.sup.2                                                     ______________________________________                                        C.sub.5 H.sub.11  C.sub.2 H.sub.5                                             C.sub.11 H.sub.23 C.sub.2 H.sub.5                                             C.sub.8 H.sub.17  C.sub.4 H.sub.9                                             C.sub.8 H.sub.11  CH.sub.3                                                    C.sub.8 H.sub.17  C.sub.2 H.sub.4OC2H.sub.5                                   C.sub.5 H.sub.11  C.sub.2 H.sub.4OC2H.sub.5                                   C.sub.11 H.sub.23 C.sub.2 H.sub.4OC2H.sub.5                                   ______________________________________                                    

The examples below relate to ferroelectric liquid-crystalline media.

EXAMPLE A

A liquid-crystalline medium is prepared which comprises:

5.8% of (2-hexyloxyphenyl)-5-heptylpyrimidine

5.8% of (2-octyloxyphenyl)-5-heptylpyrimidine

5.8% of (2-decyloxyphenyl)-5-heptylpyrimidine

1.1% of (2-p-octyloxyphenyl)-5-octylpyrimidine

1.1% of (2-p-nonyloxyphenyl)-5-octylpyrimidine

1.1% of (2-p-decyloxyphenyl)-5-octylpyrimidine

7.0% of 2-(4-octyloxyphenyl)-5-octyloxy-6-fluoropyridine

4.1% of 2-(4-heptyloxyphenyl)-5-dodecyloxy-6-fluoropyridine

4.1% of 2-(4-decyloxyphenyl)-5-undecyloxy-6-fluoropyridine

5.0% of 2-(4-octyloxy-2,3-difluorophenyl)-5-nonylpyrimidine

5.0% of 2-(4-heptyloxy-2,3-difluorophenyl)-5-nonylpyrimidine

5.0% of 2- (4-nonyloxy-2,3-difluorophenyl ) -5-nonylpyrimidine

18.0% of 2- (4-hexyloxyphenyl ) -5-hexyloxypyrimidine and

10% of optically active2-[4-(2-fluorooctyloxy)-2,3-difluorophenyl]-5-heptylpyrimidine.

This medium has a broad S_(c) * phase range and high spontaneouspolarization.

EXAMPLE B

A liquid-crystalline medium is prepared which comprises:

5.8% of 2 -(p-hexyloxyphenyl)-5-heptylpyrimidine

5.8% of 2 -(p-octyloxyphenyl)-5-heptylpyrimidine

5.8% of 2 -(p-decyloxyphenyl)-5-heptylpyrimidine

1.1% of 2 -(p-octyloxyphenyl)-5-octylpyrimidine

1.1% of 2-(p-nonyloxyphenyl)-5-octylpyrimidine

2.2% of 2-(p-decyloxyphenyl)-5-octylpyrimidine

5.0% of 2-(4-decyloxyphenyl)-5-dodecyloxy-6-fluoropyrimidine

10.1% of 2-(4-octyloxyphenyl-2-yl)-5-octyloxy-6-fluoropyrimidine

10.1% of 2-(5-decyloxypyridin-2-yl)-5-undecyloxypyrimidine

10% of 2-(4-octyloxy-2,3-difluorophenyl)-5-heptylpyrimidine

5% of 2-(4-octyloxy-2,3-difluorophenyl)-5-nonylpyrimidine

5% of 2-(4-heptyloxy-2,3-difluorophenyl)-5-nonylpyrimidine

5% of 2-(4-nonyloxy-2,3-difluorophenyl)-5-nonylpyrimidine

18% of 2-(p-hexyloxyphenyl)-5-hexyloxypyrimidine and

10% of optically active2-[4-(2-fluorooctyloxy)phenyl]-5-octyloxypyridine

This medium has an S_(c) * phase range of greater than 60° C. and highspontaneous polarization.

EXAMPLE C

To an achiral base mixture comprising

3.3% of 2-(4-hexyloxyphenyl)-5-heptylpyrimidine

3.3% of 2-(4-heptyloxyphenyl)-5-heptylpyrimidine

3.3% of 2-(4-octyloxyphenyl)-5-heptylpyrimidine

3.3% of 2-(4-nonyloxyphenyl)-5-heptylpyrimidine

7.7% of 2-(4-hexyloxyphenyl)-5-nonylpyrimidine

25.3% of 2-(4-nonyloxyphenyl)-5-nonylpyrimidine

30.8% of 4-(4'-octylbiphenyl-4-yl)-1-cyano-1-butylcyclohexane

15.4% of 4-(4'-heptylbiphenyl-4-yl)-1-cyano-1-hexylcyclohexane

6.6% of 1,4-bis[trans-4-pentylcyclohexyl]-1-cyanocyclohexane

which has the following phase transitions:

    S.sub.c 76 S.sub.A 80 N 36

are added 10% of chiral2-(4-octylphenyl)-5-(2-fluorooctyloxy)-6-fluoropyridine.

The ferroelectric medium exhibits:

    S.sub.c * 73 S.sub.A 78 CN 91 I

Spontaneous polarization (at 20° C.): 17.6 nC.cm⁻¹

Response time (at 20° C. and 15 V/μm): 85 μsec

EXAMPLE 10 10A 2-Propyn-1-yl-6-fluoropyridine

A mixture of 0.06 mol of propynylmagnesiumbromide and 25 ml of THF isadded at 40°-40° C. [sic] to a mixture of 0.05 mol of 2A and 15 ml ofTHF. The mixture is stirred at room temperature for 2 hours andsubjected to customary work-up. The unpurified product is processedfurther as in 10B.

10B 2-Propynyl-6-fluoropyridin-5-ylboric acid

0.05 mol of 10A, 0.08 mol of LDA and 0.08 mol of trimethyl borate arereacted with one another analogously to Example 1B. Aqueous work-upgives the unpurified product, which is processed further as in 10C.

10C 2-Propyl-6-fluoro-5-[4-(trans-4-pentylcyclohexyl)phenyl]pyridine

0.02 mol of 10B is coupled with 0.02 mol of4-(trans-4-pentylcyclohexyl)bromobenzene as in EP 0 354 434 in thepresence of sodium bicarbonate andtetrakis[triphenylphosphine]palladium. Conventional work-up andpurification by chromatography gives the alkyne intermediate as acolorless solid, which is hydrogenated using Pd charcoal in THF to givethe alkyl compound, C 66 N 123 I.

The following are prepared analogously:

    ______________________________________                                         ##STR78##                                                                     R.sup.1                                                                              ##STR79##    n                                                                                    ##STR80##  R.sup.2                                ______________________________________                                        C.sub.3 H.sub. 7                                                                      ##STR81##   1                                                                                     ##STR82## C.sub.3 H.sub.7                         C.sub.5 H.sub.11                                                                      ##STR83##   1                                                                                     ##STR84## C.sub.3 H.sub.7                         C.sub.5 H.sub.11                                                                      ##STR85##   1                                                                                     ##STR86## C.sub.5 H.sub.11                        C.sub.3 H.sub.7                                                                       ##STR87##   0      --         C.sub.5 H.sub.11                        C.sub.3 H.sub. 7                                                                      ##STR88##   0      --         C.sub.3 H.sub.7                         C.sub.5 H.sub.11                                                                      ##STR89##   0      --         C.sub.3 H.sub.7                         C.sub.5 H.sub.11                                                                      ##STR90##   0      --         C.sub.5 H.sub.11                        C.sub.8 H.sub.17                                                                      ##STR91##   0      --         C.sub.8 H.sub.17                        C.sub.8 H.sub.17                                                                      ##STR92##   0      --         C.sub.10 H.sub.21                       C.sub.8 H.sub.17                                                                      ##STR93##   0      --         OC.sub.8 H.sub.17                       C.sub.10 H.sub.21                                                                    0##STR94##   O      --         OC.sub.10 H.sub.21                      C.sub.3 H.sub.7                                                                       ##STR95##   1                                                                                     ##STR96## C.sub.5 H.sub.11                        C.sub.3 H.sub.7                                                                       ##STR97##   1                                                                                     ##STR98## C.sub.3 H.sub.7                         C.sub.5 H.sub.11                                                                      ##STR99##   1                                                                                     ##STR100##                                                                              C.sub.3 H.sub.7                         C.sub.5 H.sub.11                                                                      ##STR101##  1                                                                                     ##STR102##                                                                              C.sub.5 H.sub.11                        ______________________________________                                    

EXAMPLE D

A liquid-crystalline medium is prepared which comprises:

20.00% of 2-(p-heptyloxyphenyl)-5-nonylpyrimidine

20.00% of 2-(p-octyloxyphenyl)-5-nonylpyrimidine

20.00% of 2-(p-nonyloxyphenyl)-5-nonylpyrimidine

20.00% of 2-(p-hexyloxyphenyl)-5-hexyloxypyrimidine

6.00 % of 2-(p-octyloxyphenyl )-6-fluoro-5-octyloxypyrimidine

6.00 % of 2-(p-octyloxyphenyl )-6-fluoro-5-decyloxypyrimidine

6.66 % of 2-(p-octyloxyphenyl )-6-fluoro-5-octyloxypyrimidine

This medium has the following phase transistions at C 5 S_(c) 69 Ch 74I.

EXAMPLE E

A ferroelectric, liquid-crystalline medium is prepared which comprises:

19.00% of 2-(p-heptyloxyphenyl)-5-nonylpyrimidine

19.00% of 2-(p-octyloxyphenyl)-5-nonylpyrimidine

19.00% of 2-(p-nonyloxyphenyl)-5-nonylpyrimidine

19.00% of 2-(p-hexyloxyphenyl)-5-hexyloxypyrimidine

6.34% of 2-(4-heptyloxy-2,3-difluorophenyl)-5-nonylpyrimidine

6.34% of 2-(4-octyloxy-2,3-difluorophenyl)-5-nonylpyrimidine

6.33% of 2-(4-nonyloxy-2,3-difluorophenyl)-5-nonylpyrimidine

5.00% of 2-(p-octylphenyl)-6-fluoro-5-(2-fluorooctyloxy)pyridine

This medium has the following physical properties:

C 5 S_(c) * 63 S_(A) 67 Ch 71 I

P_(s) (20° C.): 6.8 nC.cm⁻²

τ (20° C./15 Vμm⁻¹): 84 μs

EXAMPLE F

A nematic liquid-crystalline ECB medium is prepared which comprises:

9.0% of p-(trans-4-propylcyclohexyl)-methoxybenzene

9.0% of p-(trans-4-propylcyclohexyl)-ethoxybenzene

9.0% of p-(trans-4-propylcyclohexyl)-butoxybenzene

5.0% of 4-ethyl-4'-methoxytolan

5.0% of 4-methyl-4'-ethoxytolan

4.0% of 2,2',3,3'-tetrafluoro-4-butoxy-4'-propyltolan

4.0% of 2,2',3,3'-tetrafluoro-4-butoxy-4'-pentyltolan

13.0% of 2,3-difluoro-4-ethoxy-4'-propyltolan

13.0% of 2,3-difluoro-4-ethoxy-4'-pentyltolan

5.0% of 4-(trans-4-propylcyclohexyl)-4'-methoxytolan

5.0% of 4-(trans-4-propylcyclohexyl)-4'-ethoxytolan

5.0% of 4-(trans-4-propylcyclohexyl)-4'-propoxytolan

2.0% of2,3-difluoro-4-ethoxyphenyltrans,trans-4-propylcyclohexylcyclohexane-4'-carboxylate7.0% of2-(trans,trans-4-propylcyclohexylcyclohexan-4'-yl)-6-fluoro-5-ethoxypyridine5.0% of 1-(p-pentylphenyl)-2-(6-fluoro-5-ethoxypyridin-2-yl)ethyne

EXAMPLE G

A nematic liquid-crystalline ECB medium is prepared which comprises:

9.0% of p-(trans-4-propylcyclohexyl)-methoxybenzene

9.0% of p-(trans-4-propylcyclohexyl)-ethoxybenzene

9.0% of p-(trans-4-propylcyclohexyl) -butoxybenzene

5.0% of 4-ethyl-4'-methoxytolan

5.0% of 4-methyl-4'-ethoxytolan

4.0% of 2,2',3,3'-tetrafluoro-4-butoxy-4'-propyltolan

4.0% of 2,2',3,3'-tetrafluoro-4-butoxy-4'-pentyltolan

13.0% of 2,3-difluoro-4-ethoxy-4'-propyltolan

13.0% of 2,3-difluoro-4-ethoxy-4 '-pentyltolan

5.0% of 4-(trans-4-propylcyclohexyl)-4'-methoxytolan

5.0% of 4-(trans-4-propylcyclohexyl)-4'-ethoxytolan

5.0% of 4-(trans-4-propylcyclohexyl)-4'-propoxytolan

6.0% of 4-(trans,trans-4-propylcyclohexylcyclohexan-4'-yl)-2,3-difluoroethoxybenzene

5.0% of 4'-(trans-4-pentylcyclohexyl)-2,3-difluoro-4-ethoxybiphenyl

3.0% of 2-(p-pentylphenyl)-6-fluoro-5-ethoxypyridine

EXAMPLE H

A nematic liquid-crystalline ECB medium is prepared which comprises:

9.0% of p-(trans-4-propylcyclohexyl)-methoxybenzene

9.0% of p-(trans-4-propylcyclohexyl)-ethoxybenzene

9.0% of p-(trans-4-propylcyclohexyl)-butoxybenzene

5.0% of 4-ethyl-4'-methoxytolan

5.0% of 4-methyl-4'-ethoxytolan

4.0% of 2,2',3,3'-tetrafluoro-4-butoxy-4'-propyltolan

4.0% of 2,2',3,3'-tetrafluoro-4-butoxy-4'-pentyltolan

13.0% of 2,3-difluoro-4-ethoxy-4'-propyltolan

13.0% of 2,3-difluoro-4-ethoxy-4'-pentyltolan

5.0% of 4-(trans-4-propylcyclohexyl)-4'-methoxytolan

5.0% of 4-(trans-4-propylcyclohexyl)-4'-ethoxytolan

5.0% of 4-(trans-4-propylcyclohexyl)-4'-propoxytolan

3.0% of 2,3-difluoro-4-ethoxyphenyltrans,trans-4-propylcyclohexylcyclohexan-4'-ylcarboxylate

5.0% of4-(trans,trans-4-propylcyclohexylcyclohexan-4'-yl)-2,3-difluoroethoxybenzene

4.0% of 2-(p-pentylphenyl)-6-fluoro-5-ethoxypyridine

The physical properties of the media for ECB displays of Examples F, Gand H are shown in Table I below:

                  TABLE I                                                         ______________________________________                                        Example       F          G        H                                           ______________________________________                                        Clearing point (°C.)                                                                 +83        +84      +83                                         Viscosity (mm.sup.2 s.sup.-1)                                                               25         <25      <25                                         at 20° C.                                                              Δn      +0.218     +0.217   +0.214                                      V.sub.(90,0,20) (V)                                                                         2.4        2.8      2.8                                         V.sub.(10,0,20) (V)                                                                         2.6        2.9      2.9                                         ______________________________________                                    

We claim:
 1. 3,6-Disubstituted 2-halopyridines of the formula I##STR103## where and R^(1') are each, independently of one another, analkyl or alkenyl radical having up to 15 carbon atoms which isunsubstituted or monosubstituted by CN, halogen or CF₃, it also beingpossible for one or more CH₂ groups in these radicals to be replaced, ineach case independently of one another, by --S--, --O--,--CO--,--CO--O--,--O--CO-- or --O--CO--O-- in such a manner that Sand/or O atoms are not linked directly to one another, and one of theradicals R¹ and R² is alternatively halogen, --CF₃, --OCF₃ or OCHF₂,A¹and A² are each, independently of one another, a(a)trans-1,4-cyclohexylene radical in which, in addition, one or morenon-adjacent CH₂ groups may be replaced by --O-- and/or --S--, (b)1,4-phenylene radical in which, in addition, one or two CH groups may bereplaced by N, (c) radical from the group comprising 1,3 -cyclobutylene,1,3-bicyclo[1.1.1]pentylene, 1,4-cylohexenylene,1,4-bicyclo[2.2.2]octylene, piperidine-1,4-diyl, naphthalene-2,6 -diyl,decahydronaphthalene-2,6-diyl and1,2,3,4-tetrahydronaphthalene-2,6-diyl, it being possible for theradicals (a) and (b) to be substituted by CN or halogen, Z¹ and Z² areeach, independently of one another, --CH₂ --CH₂ --, --C--C--, --CH₂₀ --,--OCH₂ --,--CO--O--, --O--CO--, --CH═N--, --CH₂ S--, --SCH₂ --, a singlebond or an alkylene group having 3 to 6 carbon atoms in which, inaddition, one CH₂ group may be replaced by --O--,--CO--O--, --O--CO--,--CH-halogen- or --CHCN--,L is F, m is 0, 1 or 2, n is 0, 1 or 2, andm+n is 1, 2 or 3,with the proviso that Z² is --CH₂ CH₂ --, --C--C--,--CH₂ O--, --CO--O-- or a single bond.
 2. Halopyridines according toclaim 1, in whichn is 1 or 2, and at least one of the radicals A¹ and A²is optionally fluorine-substituted 1,4-phenylene, 1,4-cyclohexylene,pyrimidine-2,5-diyl or pyridine-2,5-diyl.
 3. Liquid-crystalline mediumhaving at least two components, characterized in that at least onecomponent is a compound of the formula I of claim
 1. 4. Electroopticaldisplay, comprising as a dielectric material, a liquid-crystallinemedium according to claim
 6. 5. Ferroelectric crystalline medium havingat least one chiral component and at least one achiral component,characterized in that at least one component contains aliquid-crystalline compound which contains a group of the formula##STR104## in which L is F, as a structural element, characterized inthat it contains at least one compound of the formula I according toclaim
 1. 6. Ferroelectric crystalline medium having at least one chiralcomponent and at least one achiral component, characterized in that atleast one component contains a liquid-crystalline compound whichcontains a group of the formula ##STR105## in which L is F, as astructural element, characterized in that it contains, as dielectric, aliquid-crystalline medium according to claim
 3. 7. A ferroelectricliquid crystalline medium as in claim 3, wherein L is F.
 8. Aferroelectric liquid crystalline medium as in claim 3, additionallycomprising a phenyl pyrimidine of the formula ##STR106## wherein R' andR" are each C₂₋₁₀ -straight-chain alkyl or alkoxy groups.
 9. Aferroelectric liquid crystalline medium as in claim 7, comprising acompound of formula I ##STR107## wherein A¹ and A² are each,independently of one another, a(a) trans-1,4-cyclohexylene radical inwhich, in addition, one or more non-adjacent CH₂ groups may be replacedby --O-- and/or --S--, (b) 1,4-phenylene radical in which, in addition,one or two CH groups may be replaced by N, (c) radical from the groupcomprising 1,3-cyclobutylene, 1,3-bicyclo[1.1.1]pentylene,1,4-cyclohexenylene, 1,4-bicyclo[2.2.2]octylene, piperidine-1,4-diyl,naphthalene-2,6-diyl, decahydronaphthalene-2,6-diyl and1,2,3,4-tetrahydronaphthalene-2,6-diyl it being possible for theradicals (a) and (b) to be substituted by CN or halogen, Z¹ and Z² areeach, independently of one another, --CH₂ --CH₂ --, --CH₂ O--, --OCH₂--, --CO--O--, --O--CO--, --CH═N--, --CH₂ S--, --SCH₂ --, a single bondor an alkylene group having 3 to 6 carbon atoms in which, in addition,one CH₂ group may be replaced by --O--, --CO--O--, --O--CO--,--CH-halogen- or --CHCN--, L is F, m is 0, 1 or 2, n is 0, 1 or 2, andm+n is 1, 2 or 3,with the proviso that where R² is halogen, CN, --CF₃,OCF₃, OCHF₂ or alkoxy, n is 1 or 2, and Z² is --CH₂ --CH₂ --,--C.tbd.C--, --CH₂ O--, --CO--O-- or a single bond, wherein one of theradicals R¹ and R² is a chiral group selected from the group consistingof compounds of formulae III, Ilia, and IIIc, wherein formula III is asfollows: ##STR108## wherein R⁴ is a group of the formula (CH₂)_(n) --Q³--C_(o) H_(2no+1), in which Q³ is --O--, --O--CO--, or a single bond; nis 0, 1 or 2; and o is 1 to 7; Y is CN, halogen, or CH₃ ; Z is a singlebond or --(CH₂)_(p) --, in which one CH₂ group may be replaced by --O--,--O--CO--, or --CO--O--, and p is 1, 2, 3, 4, 5, or 6; and R^(o) is H orCH₃,with the proviso that only one of the groups R^(o), R⁴, and Y is CH₃; wherein formula Ilia is as follows: ##STR109## wherein o is I to 7; Q³and Q⁴ are each independently --O--, --CO--O--, or a single bond; s is 1or 2; and p is 0 or 1; and wherein formula IIIc is as follows:##STR110## in which Q³ and Q⁴ are each independently --O--, --CO--O--,or a single bond; r is 1 or 2; and p is 0 or
 1. 10. 3,6-Disubstituted2-halopyridines of the formula I' ##STR111## wherein R¹ and R² are each,independently of one another, an alkyl or alkenyl radical having up to15 carbon atoms which is unsubstituted or monosubstituted by CN, halogenor CF₃, it also being possible for one or more CH₂ groups in theseradicals to be replaced, in each case independently of one another, by--S--, --O--, --CO--, --CO--O--, --O--CO-- or --O--CO--O-- in such amanner that S and/or O atoms are not linked directly to one another,A¹and A² are each, independently of one another, a(a)trans-1,4-cyclohexylene radical in which, in addition, one or morenon-adjacent CH₂ groups may be replaced by --O-- and/or --S--, (b)1,4-phenylene radical in which, in addition, one or two CH groups may bereplaced by N, it being possible for the radicals (a) and (b) to besubstituted by CN or halogen, Z¹ and Z² are each, independently of oneanother, --CH₂ --CH₂ --, --C.tbd.C--, --CH₂ O--, --OCH₂ --, --CO--O--,--O--CO--, --CH═N--, --CH₂ S--, --SCH₂ --, a single bond or an alkylenegroup having 3 to 6 carbon atoms in which, in addition, one CH₂ groupmay be replaced by --O--, --CO--O--, --O--CO--, --CH-halogen- or--CHCN--, L is F m is 0, 1 or 2, n is 1 or 2, and m+n is 1, 2 or
 3. 11.A liquid crystalline medium having at least two components, wherein atleast one component is a compound of formula I' of claim
 10. 12. Amatrix liquid crystal display which contains, as a dielectric, a liquidcrystal medium according to claim
 11. 13. A liquid crystal switching anddisplay device containing a ferroelectric liquid crystalline medium,outer plates, electrodes, at least one alignment layer and, optionally,additional assistant layers which contain at least one liquid crystalcompound of formula I' of claim 10.